I think Yamato's cuffs prevented her from transforming.
Okay, there are several reasons to think Yamato's Mythical is Byakko.
Here's her DF hint:
She is a carnivorous Zoan with pronounced top canines.
She also
growls.
Growling is most commonly associated with felines and canines:
This is a strong hint that Yamato is a feline or canine.
One of her favourite foods is eating salmon raw.
This might suggest feline (or just be something she inherited from Kaido).
A bear also fits, but the colour scheme wouldn't work unless it's a Polar Bear, and ordinary bears are way too lame for Yamato's DF.
She has a White - teal colour scheme:
Her colours would fit well with popular renditions of the Byakko:
She's waging a very personal battle against Kaido.
Byakko is the mortal enemy of Seiryu and Kaido is the mortal enemy of Yamato.
(In contrast, Western and Eastern Dragons have no such antagonistic relationship).
Western Dragon is my second choice after Byakko, but I think Byakko fits best.
I think something like:
>= 80% that Yamato has a Mythical Zoan.
>= 60% that Yamato has Neko Neko no Mi: Model Byakko (or equivalent).